The present invention relates to protective coatings for components exposed to high temperatures, such as components of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating system that includes a thermal-insulating ceramic layer with improved stability and impact-resistance at elevated temperatures, and lower density and thermal conductivity.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature alloys have been achieved through the formulation of iron, nickel and cobalt-base superalloys, though components formed from such alloys often cannot withstand long service exposures if located in certain sections of a gas turbine engine, such as the turbine, combustor or augmentor. A common solution is to protect the surfaces of such components with a protective coating system, such as an aluminide coating or a thermal barrier coating (TBC) system. The latter includes an environmentally-resistant bond coat and a thermal barrier coating (TBC) of a ceramic material applied as a topcoat over the bond coat. Bond coats are typically formed of an oxidation-resistant alloy such as MCrAlY where M is iron, cobalt and/or nickel, or a diffusion aluminide or platinum aluminide. During high temperature excursions, these bond coats form an oxide layer or scale that chemically bonds the ceramic layer to the bond coat.
Zirconia (ZrO2) that is partially or fully stabilized by yttria (Y2O3), magnesia (MgO), calcia (CaO), ceria (CeO2) or other oxides has been employed as the ceramic material for the TBC. On occasion, combinations of oxides have been employed, as represented by U.S. Pat. No. 4,774,150 (zirconia stabilized by yttria, calcia and/or magnesia +Bi2O3, TiO2, Tb4O7, Eu2O3 or Sm2O3 as a xe2x80x9cluminous activatorxe2x80x9d). Yttria-stabilized zirconia (YSZ) is widely used as the insulating layer for TBC systems used in demanding applications because it exhibits desirable thermal cycle fatigue properties. In addition, YSZ can be readily deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), and physical vapor deposition (PVD) techniques such as electron beam physical vapor deposition (EBPVD). Notably, YSZ deposited by EBPVD is characterized by a strain-tolerant columnar grain structure, enabling the substrate to expand and contract without causing damaging stresses that lead to spallation.
As is known in the art, stabilization inhibits zirconia from undergoing a phase transformation (tetragonal to monoclinic) at about 1000xc2x0 C. that would otherwise result in a detrimental volume expansion. Traditionally, zirconia for TBC systems has been stabilized with at least six weight percent yttria to avoid this transformation. In S. Stecura, xe2x80x9cEffects of Compositional Changes on the Performance of a Thermal Barrier Coating System,xe2x80x9d NASA Technical Memorandum 78976 (1976), tests showed that plasma sprayed YSZ coatings containing six to eight weight percent yttria were more adherent and resistant to high temperature thermal cycling than YSZ coatings containing greater and lesser amounts of yttria. Contrary to the teachings of Stecura, in commonly-assigned U.S. Pat. No. 5,981,088 to Bruce et al., it was unexpectedly shown that if a YSZ coating has a columnar grain structure (e.g., deposited by EBPVD), superior spallation resistance can be achieved if zirconia is partially stabilized by less than six weight percent yttria. Significantly, YSZ TBCs in accordance with Bruce et al. contain the monoclinic phase, which was intentionally avoided in the prior art by the six to eight weight percent yttria advocated by Stecura.
In addition to being resistant to spallation from thermal cycling, a thermal barrier coating on a gas turbine engine component is required to withstand damage from impact by hard particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The result of impingement can be erosive wear (generally from smaller particles) or impact spallation from larger particles. Accordingly, in addition to the greater spallation resistance from thermal cycling achieved with the teachings of Bruce et al., further improvements are desired for TBC materials. For example, greater impact resistance would also be desirable, as well as such other improvements such as lower density and lower thermal conductivity for more demanding applications at higher temperatures.
The present invention provides a thermal barrier coating (TBC) system for components designed for use in a hostile thermal environment, such as the turbine, combustor and augmentor sections of a gas turbine engine. The TBC system is particularly suited for applications in which temperatures in excess of about 2000xc2x0 F. (about 1090xc2x0 C.) are encountered and induce severe thermal cycle fatigue stresses. The TBC system employs a thermal-insulating ceramic topcoat, or TBC, that is compatible with known metallic bond coats, such as diffusion aluminides and MCrAlY and NiAl coatings.
In particular, a TBC material in accordance with this invention is formed of zirconia partially stabilized by about one up to less than six weight percent yttria and further stabilized by about one to about ten weight percent of magnesia and/or hafnia. Furthermore, the TBC preferably has a columnar grain structure of the type produced when deposited by a PVD technique, preferably EBPVD.
TBC systems with the TBC material of this invention have been surprisingly shown to exhibit superior impact resistance as compared to conventional YSZ coatings containing more than six weight percent yttria. Importantly, prior art TBC systems have avoided zirconia stabilized by less than six weight percent yttria because such materials undergo a phase transformation that promotes spallation of the ceramic layer. Furthermore, prior art zirconia-based materials for TBC systems have typically been limited to a single stabilizing additive, primarily yttria or magnesia. However, in accordance with this invention, YSZ containing low levels of yttria in combination with magnesia and/or hafnia have been determined to exhibit improved impact resistance.
Other objects and advantages of this invention will be better appreciated from the following detailed description.